The present invention relates to a portable communication device such as a cellular phone, and more particularly to a dual-mode communication system having a simplified circuit construction for performing digital and analog communication.
With the ever increasing popularity of cellular telephony, it has become necessary to provide more channels in the allocated cellular frequencies. Up until recently, existing cellular telephone systems have been comprised exclusively of xe2x80x9canalog voicexe2x80x9d systems. In this type of system, an analog voice signal is used to modulate a radio frequency (RF) carrier or intermediate frequency (IF) signal. Recently, xe2x80x9cdigital voicexe2x80x9d systems have been developed in which analog communication signals are processed using digital signal processing techniques. In this type of system, the analog voice signal is first digitized and is then used to modulate either an RF carrier or an IF signal. The analog voice systems are designed for performing a frequency modulation (FM) mode of an advanced mobile phone service (AMPS), and the digital voice systems are designed for performing a time division multiple access (TDMA) mode and a code division multiple access (CDMA) mode.
More recently, the cellular telephone industry is developing xe2x80x9cdual-modexe2x80x9d systems. A typical type of the dual-mode system is designed for operation in both a digital communication mode (for example, CDMA mode) and in an analog communication mode (for example, AMPS mode).
In the digital communication mode, a cellular phone converts an analog voice signal received from a microphone into a digital signal by sampling the voice signal. The digital signal is transmitted to a digital signal processor including an encoder and a decoder. The digital signal processor encodes the digital signal in response to the digital communication mode, either the TDMA mode or the CDMA mode. The encoded digital signal is re-converted into an analog signal. The analog signal is modulated in a radio frequency (RF) receiver/transmitter with carrier. In addition, a voice signal transmitted from a base station is a compressed voice signal. The voice signal is demodulated and decoded. The decoded voice signal is decompressed and recovered into a sampled analog voice signal, which is transmitted into a speaker.
In the analog communication mode, the cellular phone converts an analog voice signal from the microphone into a digital signal, and compresses the digital signal using a compressor before its transmission. In addition, a signal transmitted from the base station is recovered through an expander. The recovered signal is converted into the analog signal and outputted through the speaker.
The compressor and the expander are used for removing channel noise. For example, if the noise contained in the voice signal passes through a channel after leveling up the noise via the compressor, the major channel noise become lower than that of the voice signal. If the signal is received and passed through the expander, the noise level of the signal becomes low to the original channel noise level. In that case, the channel noise contained to the signal is lower than the noise level, so that the noise is removed.
In the most straightforward proposed implementations of CDMA/AMPS and other dual-mode systems, separate circuitry is employed for processing digitized speech information during each mode of operation. Thus, the conventional dual-mode cellular phone systems require a higher cost and a larger area for the extra circuits required to perform each mode of operations relative to single mode systems. To solve the above problems, a novel dual-communication system employing a simplified circuitry for performing both the digital communication and the analog communication, is desired.
For constructing a dual-mode communication system, a design of a compandor including the compressor and the expander is very important. Examples of companders can be found in U.S. Pat. No. 3,919,654 to Rouben Toumani, issued on Nov. 11, 1975, xe2x80x9cSYLLABIC COMPANDORxe2x80x9d; and U.S. Pat. No. 4,355,304 to Kasuga et al., issued on Oct. 19, 1982, xe2x80x9cDIGITAL COMPANDORxe2x80x9d. In addition, a theory of operation of a syllabic compandor is set forth in xe2x80x9cTheory of syllabic compandorsxe2x80x9d, by R. O. Carter, March, 1964 PROC. IEEE, Vol. III, No. 3, pp. 503-513.
Further, standards of a compandor are determined by a level change ratio, an attack time and a recovery time. The attack time and the recovery time are illustrated in detail on Telecommunication Industry Association(TIA)/Electronic Industry Association(EIA)/International Standard(IS)-98 as an AMPS specification. Although level change ratios of a compressor and an expander are equal, received voice signal is distorted when the attack time and the recovery time are different. Thus, a need exists for a novel compandor which can readily control the attack time and the recovery time.
It is therefore an object of the present invention to provide a dual-mode communication system having a digital signal processor for performing an analog communication as well as a digital communication.
It is another object of the invention to provide a signal processing method of the dual-mode communication system for readily controlling an attack time and a recovery time of the system.
It is still another object of the invention to provide a dual-mode communication system having a simplified circuit construction.
According to an aspect of the present invention, there is provided a communication system including a compressor for compressing a voice signal in response to a predetermined communication mode and an expander for expanding the compressed voice signal. The compressor includes a divider for dividing the voice signal by a first gain, a first full-wave rectifier for rectifying the divided signal from the divider, and a first gain controller for updating the first gain in response to the rectified signal so as to generate a compressed voice signal. The expander includes a second full-wave rectifier for rectifying the compressed voice signal, a second gain controller for updating a second gain in response to the rectified signal, and a multiplier for multiplying the compressed voice signal and the updated second gain so as to expand the compressed voice signal.
The compressor may further include a first filter coupled between the first full-wave rectifier and the first gain controller, for smoothing the rectified signal from the first full-wave rectifier. The first filter may be a digital infinite impulse response (IIR) filter which can control an attack time and a recovery time of the compressor in response to a time constant of the first filter. The expander may further include a second filter coupled between the second full-wave rectifier and the second gain controller, for smoothing the rectified signal from the second full-wave rectifier. The second filter may be a digital IIR filter which can control an attack time and a recovery time of the expander in response to a time constant of the second filter. The voice signal may be transmitted and received in an analog communication mode, for example, an advanced mobile phone service (AMPS) mode. The compressor and the expander may be included in a digital signal processor which compresses and expands a voice signal transmitted and received in a digital communication mode, for example, a code division multiple access (CDMA) mode. The communication system may also include a signal processor for converting a digital signal from the compressor into an analog signal, a transmitter for transmitting the analog signal from the signal processor with a carrier to a base station, and a receiver for receiving a compressed analog signal from the base station, wherein the signal processor converts the compressed analog signal from the receiver into a digital signal, for providing the digital signal to the expander.
According to another aspect of this invention, there is provided a method for processing a voice signal from/to a communication system including the steps of dividing the voice signal by a first gain, calculating a first magnitude value of the divided signal, and updating the first gain in response to the calculated first magnitude value, so as to compress the voice signal. The method further includes the steps of calculating a second magnitude value of a compressed voice signal, updating a second gain in response to the second magnitude value, and multiplying the compressed voice signal and the updated second gain, so as to expand the compressed voice signal. The step of calculating a first magnitude value may include performing a full-wave rectification of the divided voice signal. The step of calculating a second magnitude value may also include performing a full-wave rectification of the compressed voice signal.
The method of present invention may also include the steps of performing a resistor-capacitor (RC) filtration of the calculated first magnitude value, and performing a RC filtration of the calculated second magnitude value. The divided voice signal in the step of dividing may be directly proportional to an about square root of the voice signal before the step of dividing. The compressed voice signal multiplied in the step of multiplying may be directly proportional to an about square of the compressed voice signal in the step of calculating a second magnitude value.